The studies to be carried out are: (1) A comparison of the rates of weight gain of the fetal lamb during the final weeks of gestation and during the first three weeks postpartum while it is nursing on its mother, to establish the degree, if any, to which the growth of the fetus is limited by the availability of nutrients via the placenta; (2) the relation between the rate of weight gain (gm/day) of individual fetal lambs during the final weeks of gestation, the oxygen consumption of the uterus and its contents (ml/kg/min), and the rate at which they take up amino acids and glucose; (3) the effects of ovariectomy of the ewe on or about the 100th day of gestation, on the rates of uterine blood flow, oxygen consumption and amino acid uptake as well as the levels of estrogens and progesterones in the maternal blood and their rates of release into the maternal circulation by the chorion; (4) the effects of "high" and "low" daily doses of progesterone on the weights of the uterus, placenta and fetus as well as on the volumes of the allantoic and amniotic fluids after the 60th day of gestation in ewes castrated on the fourth day after conception.